How Do i export to android properly?

How Do I export to android properly? I have the android sdk linked with unity but i get a error
saying NullReferenceException: Object reference not set to an instance of an object
UnityEditor.Android.PostProcessAndroidPlayer…ctor ()
UnityEditor.Android.PostProcessAndroidPlayer.PostProcess (BuildTarget target, System.String stagingAreaData, System.String stagingArea, System.String playerPackage, System.String installPath, System.String companyName, System.String productName, BuildOptions options)
UnityEditor.Android.AndroidBuildPostprocessor.PostProcess (BuildPostProcessArgs args)
UnityEditor.PostprocessBuildPlayer.Postprocess (BuildTarget target, System.String installPath, System.String companyName, System.String productName, Int32 width, Int32 height, System.String downloadWebplayerUrl, System.String manualDownloadWebplayerUrl, BuildOptions options, UnityEditor.RuntimeClassRegistry usedClassRegistry) (at C:/buildslave/unity/build/Editor/Mono/BuildPipeline/PostprocessBuildPlayer.cs:316)
UnityEditor.HostView:OnGUI()

and heres the next one

Error building Player: NullReferenceException: Object reference not set to an instance of an object

please help

After I opened the SDK manager and grabbed ALL the tools, it finally built an .apk for me.